Or a separate species of large eel with a small population. Either way, those explanations may not be as sexy as a plesiosaur, but are more plausible.
The lack of a species of giant eel being known to science is against that possibility, as opposed to rare mutant eels that grow to giant size, which are known to science. Then again, the the murky deeps of the oceans and large fresh water lakes continue to hold mysteries. Like you, I surmise, I hope that there is a species of giant eel yet to be discovered.